Mixers, modulators and other frequency translation devices (FTDs) are ubiquitous in communication systems. Delay characteristics of FTDs influence the performance of communication systems in which the FTDs are included. For example, FTDs having nonlinear delay characteristics introduce distortion that can increase bit error rate of a communication system or otherwise corrupt integrity of information transmitted through the system. Modern communication systems that have high modulation bandwidth and complex modulation formats are especially influenced by the delay characteristics of FTDs. Accurate measurements of a FTD""s delay characteristics are necessary to predict performance of a communication system. In addition, once delay of a FTD is accurately characterized, compensation for delay nonlinearities can be provided to improve performance of the communication system in which the FTD is included.
Measuring delay characteristics of a FTD using traditional techniques is time consuming. In a technique disclosed by Clark et al. in U.S. Pat. No. 5,937,006, three transmission measurements are performed on three pairs of interchanged FTDs to extract delay characteristics of a designated one of the FTDs. Performing multiple transmission measurements and interchanging FTDs as required by this technique is time consuming. In an alternative technique, delay characteristics of a FTD are obtained from phase comparison of an input modulated signal to a demodulated signal resulting at the output of the FTD. This modulation/demodulation technique is time consuming because phase comparisons are necessary at each of the frequencies at which the delay of the FTD is characterized. In addition, accuracy of this technique is limited by an inherent trade-off between resolution of the frequencies at which delay is characterized and time resolution of the resulting delay characterization.
In view of these known techniques, there is a need for a method for characterizing delay of FTDs that is accurate and quick to perform.
A method for characterizing delay of frequency translation devices (FTDs) constructed according to the preferred embodiments of the present invention is accurate and quick to perform. Absolute delay of a FTD is characterized by a method constructed according to a first embodiment of the present invention. In this first preferred embodiment, a stimulus signal is applied to a first port of the FTD. A second port of the FTD is coupled to a delay element having a known delay and a reflective termination. A drive signal is applied to a third port of the FTD. A time domain reflection response to the stimulus signal is obtained and a signal peak within the response, that corresponds to a return signal from the reflective termination, is identified. Absolute delay of the frequency translation device is then extracted based on the known delay of the delay element and a time that corresponds to the occurrence of the identified signal peak.
Delay of a FTD versus frequency is characterized by a method constructed according to a second preferred embodiment of the present invention. In this second preferred embodiment, a stimulus signal is also applied to a first port of the FTD while the second port of the FTD is coupled to the delay element and while a drive signal is applied to the third port of the FTD. A time domain reflection response to the stimulus signal is obtained and a segment of the obtained time domain reflection response that corresponds to a return signal from the reflective termination is isolated. Inverse frequency transforming the isolated segment of the time domain reflection response provides delay characteristics of the FTD versus frequency. Inherent frequency transform/inverse frequency transform relationships between the frequency domain and the time domain enable the delay characteristics of the FTD to be provided in equivalent alternative ways in the second preferred embodiment of the present invention.